Spark plugs having an electrode, e.g., a center electrode, a noble metal tip being applied to the front end of the center electrode or a noble metal being applied circumferentially in one area, have been known in the related art for a long time.
For example, European Patent No. 0 637 113 describes a spark plug having a center electrode having a heat resistant and erosion resistant nickel alloy, the front end of the center electrode being formed to have a noble metal tip of iridium or ruthenium. The nickel alloy has a thermal conductivity of approximately 30 Wm−1K−1 or greater. This printed publication states that the noble metal tip has a discoid shape and is arranged concentrically at the front end of the electrode metal.
The use of a YAG laser, for example, causes laser beams to be applied to the noble metal tip/front end boundary zone of the electrode metal, the noble metal tip being pressed against the front end of the electrode metal to which the noble metal is to be applied using a corresponding force.
A method of manufacturing a spark plug is known from European Patent No. 0 400 950, an iridium powder blank being manufactured which forms the firing tip of the center electrode of the spark plug. This iridium powder blank is sintered in a vacuum or in a non-oxidizing or reducing atmosphere and the firing tip is joined metallurgically to the front end of the center electrode. The metallurgic joining may be accomplished, for example, using electron beam welding or laser welding.
U.S. Pat. No. 5,811,915 and German Patent Application No. 196 41 856 also describe the provision of noble metal chips on a spark plug electrode such as, for example, a ground electrode or center electrode. According to these documents, the noble metal chips are applied using laser welding, specifically with the aid of an Nd:YAG laser.
European Patent No. 0 575 163 also describes welding a noble metal chip to a center electrode of a spark plug, the weld being located at the circumference of the boundary zone between the noble metal chip and the end face of the center electrode. A YAG laser is used for the welding in this case.
U.S. Pat. No. 4,963,112 also describes the attachment of a noble metal chip to an electrode of a spark plug, the attachment being made by laser welding. It is described that pulsed lasers are preferably used.
U.S. Pat. No. 5,461,210, European Patent No. 0 588 495 and European Patent No. 0 587 446 also describe the application of noble metal chips on spark plug electrodes. In this case also, a pulsed laser beam is always used to weld on the noble metal chips.
However, all of these methods known from the related art for applying a noble metal chip or another form of a noble metal to an electrode of a spark plug have in common that the application takes place with the aid of a pulsed laser.
In joining methods of this type with the aid of pulsed laser beam sources, the material to be joined, i.e., the electrode and the noble metal, are discontinuously melted and resolidified. This means that no continuous melt is produced.
The constant melting and resolidifying of the material to be joined may, however, make a thorough mixing of the fusion zone, i.e., a uniform alloy distribution, possible only to a limited degree. For that reason, a relatively high tendency to crack arises in the weld zone, for which reason the service life of such spark plugs, which are used as “long-life spark plugs,” is ultimately limited by the relatively short fatigue endurance of the noble metal/electrode alloy join.
Frequently a nickel alloy is used as a material for the electrode. The joining using a pulsed laser beam results in undesirable alloy areas rich in nickel that are consequently less resistant to erosion and corrosion.
An observation of the surface of an electrode welded using a pulsed laser beam and provided with a noble metal shows that it is very irregular since it is not possible to produce a continuous fusion zone area but rather the material is repeatedly melted and solidified. Aftertreatment after welding may therefore be necessary.